Diving masks are composed of a pair of lenses (or a single lens) supported by a rigid frame onto whose profile the so-called face mask is fixed, formed by a sort of shroud of soft and elastically yielding material, equipped with openings for the lenses and whose rear profile is applied in an airtight way to the face of the diver thanks to a strap that surrounds the head of the diver.
However, between the face of the diver and the mask, an internal space remains which is increased by a cavity projecting to house the nose of the diver.
Therefore, the eyes and nose of the diver are closed within the mentioned face mask while his/her mouth remains outside the face mask and can be connected to a cylinder through a mouthpiece.
The mask can also be used for free diving.
In either case, although the nose is not used for breathing, it is inevitable that a small amount of damp air leaves the diver's nose and this damp air can cause the lenses to fog up, hence reducing the diver's visual ability.
Various attempts have been made to reduce the onset of lenses fogging up, mainly through special treatments of the internal surface of the lenses, the effect of which is not always long-lasting and satisfactory.